Water electrolysis system and temperature control method thereof

ABSTRACT

A water electrolysis system includes a water electrolyzer, a water supply pipe, a water supply pipe, a water discharge pipe, and a first and second temperature sensors. The water electrolyzer includes a water supply port and a water discharge port. The water supply pipe is connected to the water supply port. The water is to be supplied to the water electrolyzer via the water supply pipe. The water discharge pipe is connected to the water discharge port. The water is to be discharged from the water electrolyzer via the water discharge pipe. The first temperature sensor is provided at the water supply pipe between a cooling apparatus and the water supply port to detect a temperature of the water in the water supply pipe. The second temperature sensor is provided at the water discharge pipe to detect a temperature of the water in the water discharge pipe.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to JapanesePatent Application No. 2016-096971, filed May 13, 2016, entitled “WaterElectrolysis System and Temperature Control Method Thereof.” Thecontents of this application are incorporated herein by reference intheir entirety.

BACKGROUND 1. Field

The present disclosure relates to a water electrolysis system and atemperature control method thereof.

Hydrogen is used in general as a fuel gas to be applied to a powergeneration reaction by a fuel cell. The hydrogen is produced with useof, for example, a water electrolyzer. The water electrolyzer employs asolid polymer electrolyte membrane (ion exchange membrane) in order toproduce hydrogen (and oxygen) through electrolysis of water.Electrocatalyst layers are provided on both surfaces of the solidpolymer electrolyte membrane to form a membrane electrode assembly. Inaddition, a unit cell is constructed by providing electricity supplierson both sides of the membrane electrode assembly.

With the above configuration, a voltage is applied to both ends in astacking direction of a cell unit in which multiple unit cells arestacked. Besides, water is supplied to the electricity supplier on theanode side. Thus, water is electrolyzed to produce hydrogen ions(protons) on the anode side of the membrane electrode assembly. Thesehydrogen ions permeate the solid polymer electrolyte membrane to move tothe cathode side, and then couple with electrons to produce hydrogen.Meanwhile, on the anode side, oxygen produced together with hydrogen isdischarged from the cell unit along with surplus water.

There may be a case where, for example, a high-pressure waterelectrolyzer (high differential pressure water electrolyzer) is employedas the water electrolyzer. The high-pressure water electrolyzerproduces, through electrolysis of water, oxygen and hydrogen with apressure higher than that of the oxygen. As a high-pressure waterelectrolyzer of this kind, a water electrolysis system and an activationmethod thereof disclosed in, for example, Japanese Unexamined PatentApplication Publication No. 2015-48506 are known.

This water electrolysis system includes: a water electrolyzer (highdifferential pressure water electrolyzer); a water supply pipe whichsupplies water to the water electrolyzer; and a circulation pipe whichcirculates the water guided out of the water electrolyzer. Connected tothe water supply pipe and the circulation pipe is a gas-liquidseparation apparatus which separates a gas component contained in thewater guided out of the water electrolyzer. Connected to the watersupply pipe are: an ion exchange apparatus which includes an ionexchange unit; a circulation pump which circulates the water; and atemperature sensor which detects a temperature of the water circulatedto the water electrolyzer.

SUMMARY

According to a first aspect of the present invention, a waterelectrolysis system includes a water electrolyzer, a water supply pipe,and a water discharge pipe. The water electrolyzer electrolyzes water toproduce oxygen and hydrogen. The water supply pipe supplies the water toa water supply port of the water electrolyzer. The water discharge pipeallows the water, subjected to electrolysis, to be discharged from awater discharge port of the water electrolyzer. The water supply pipe isprovided with a cooling apparatus, and is provided with a firsttemperature sensor which is located between the cooling apparatus andthe water supply port and which monitors a temperature of supply watersupplied to the water supply port. The water discharge pipe is providedwith a second temperature sensor which monitors a temperature ofdischarge water discharged from the water discharge port.

According to a second aspect of the present invention, a temperaturecontrol method of a water electrolysis system including a waterelectrolyzer which electrolyzes water to produce oxygen and hydrogen, awater supply pipe which supplies the water to a water supply port of thewater electrolyzer, a water discharge pipe which allows the water,subjected to electrolysis, to be discharged from a water discharge portof the water electrolyzer, wherein the water supply pipe is providedwith a cooling apparatus, and is provided with a first temperaturesensor located between the cooling apparatus and the water supply port,and the water discharge pipe is provided with a second temperaturesensor, the temperature control method includes the steps of causing thesecond temperature sensor to monitor a temperature of discharge waterdischarged from the water discharge port of the water electrolyzer froma time of activation of the water electrolysis system and after thetemperature of the discharge water monitored by the second temperaturesensor exceeds a predetermined temperature, causing the firsttemperature sensor to monitor a temperature of supply water supplied tothe water supply port of the water electrolyzer, and starting control ofthe cooling apparatus to control the temperature of the supply water.

According to a third aspect of the present invention, a waterelectrolysis system includes a water electrolyzer, a water supply pipe,a water supply pipe, a cooling apparatus, a water discharge pipe, afirst temperature sensor, and a second temperature sensor. The waterelectrolyzer electrolyzes water to produce oxygen and hydrogen. Thewater electrolyzer includes a water supply port and a water dischargeport. The water supply pipe is connected to the water supply port. Thewater is to be supplied to the water electrolyzer via the water supplypipe. The cooling apparatus is provided in the water supply pipe. Thewater discharge pipe is connected to the water discharge port. The wateris to be discharged from the water electrolyzer via the water dischargepipe. The first temperature sensor is provided at the water supply pipebetween the cooling apparatus and the water supply port to detect atemperature of the water in the water supply pipe. The secondtemperature sensor is provided at the water discharge pipe to detect atemperature of the water in the water discharge pipe.

According to a fourth aspect of the present invention, a temperaturecontrol method of a water electrolysis system including a waterelectrolyzer to electrolyze water to produce oxygen and hydrogen, thetemperature control method includes detecting a temperature of water ina water discharge pipe. The water discharge pipe is connected to thewater electrolyzer. The water is discharged from the water electrolyzervia the water discharge pipe. A temperature of the water in a watersupply pipe between a cooling apparatus and the water electrolyzer isdetected after the temperature of the water detected in the waterdischarge pipe exceeds a predetermined temperature. The water supplypipe is connected to the water electrolyzer. The water is supplied tothe water electrolyzer via the water supply pipe. The cooling apparatusis started to control the temperature of the water in the water supplypipe. The cooling apparatus is provided in the water supply pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings.

FIG. 1 is a schematic configuration diagram for explanation of a waterelectrolysis system according to an embodiment of the presentdisclosure.

FIG. 2 is a flowchart for explanation of a temperature control method ofthe water electrolysis system.

FIG. 3 is a timing diagram for explanation of the temperature controlmethod.

DESCRIPTION OF THE EMBODIMENTS

The embodiments will now be described with reference to the accompanyingdrawings, wherein like reference numerals designate corresponding oridentical elements throughout the various drawings.

As illustrated in FIG. 1, a water electrolysis system 10 according to anembodiment of the present disclosure includes a high differentialpressure water electrolyzer (water electrolyzer) 12 which producesoxygen and high-pressure hydrogen through electrolysis of water (purewater). The high-pressure hydrogen refers to hydrogen with a pressureof, for example, 1 to 80 MPa, which is higher than that of atmosphericoxygen. Note that the water electrolysis system 10 is not limited to theproduction of high-pressure hydrogen, but is applicable to the case ofproducing atmospheric hydrogen.

The high differential pressure water electrolyzer 12 has a stack ofmultiple water electrolysis cells 14. At both ends in a stackingdirection of the stack of water electrolysis cells 14, end plates 16 aand 16 b are provided. Connected to the high differential pressure waterelectrolyzer 12 is an electrolytic power supply 18 being a directcurrent power supply.

The end plate 16 a is provided with a water supply port 20 a, while theend plate 16 b is provided with a water discharge port 20 b and ahydrogen outlet port 20 c. A water supply pipe 22 is connected to thewater supply port 20 a. Connected to the water supply pipe 22 are aradiator (cooling apparatus) 24 which includes a radiator fan 23, and acirculation pump 26 which circulates water. Moreover, the water supplypipe 22 is connected to a bottom portion of an oxygen gas-liquidseparation apparatus 28. Incidentally, the cooling apparatus is notlimited to radiator 24. Instead, cooling equipment of various kinds maybe used.

An upper portion of the oxygen gas-liquid separation apparatus 28communicates with a blower 30 and one end portion of a water dischargepipe 32, while the other end of the water discharge pipe 32 communicateswith the water discharge port 20 b of the high differential pressurewater electrolyzer 12. The water supply pipe 22 and the water dischargepipe 32 constitute a water circulation pipe.

Fastened to the oxygen gas-liquid separation apparatus 28 are: a purewater supply pipe 36 connected to a pure water producing unit 34; and anoxygen discharge pipe 38 for discharging oxygen (and hydrogen) separatedfrom pure water at the oxygen gas-liquid separation apparatus 28.

One end portion of a high-pressure hydrogen pipe 42 is connected to thehydrogen outlet port 20 c of the high differential pressure waterelectrolyzer 12. The other end portion of the high-pressure hydrogenpipe 42 is connected to a high-pressure hydrogen gas-liquid separationapparatus, though not illustrated. The high-pressure hydrogen gas-liquidseparation apparatus separates water from hydrogen. Thereafter, watervapor (water) contained in the hydrogen is adsorbed at a wateradsorption apparatus. As a result, a hydrogen product (dry hydrogen) isobtained.

The water electrolysis system 10 includes a controller (ECU) 44, andthis controller 44 controls water electrolysis processing (operation).

The water supply pipe 22 is provided with a first temperature sensor 46which monitors a temperature of supply water supplied to the watersupply port 20 a of the high differential pressure water electrolyzer12. The water discharge pipe 32 is provided with a second temperaturesensor 48 which monitors a temperature of discharge water dischargedfrom the water discharge port 20 b of the high differential pressurewater electrolyzer 12. Each of the first temperature sensor 46 and thesecond temperature sensor 48 sends a detected temperature signal to thecontroller 44, and the controller 44 then controls the radiator fan 23based on the detected temperature signal.

Hereinafter, a description is provided for the operations of the waterelectrolysis system 10 configured as above.

As illustrated in FIG. 1, pure water in the oxygen gas-liquid separationapparatus 28 is supplied to the water supply port 20 a of the highdifferential pressure water electrolyzer 12 via the water supply pipe 22under the action of the circulation pump 26. Meanwhile, a voltage isapplied to the high differential pressure water electrolyzer 12 usingthe electrically-connected electrolytic power supply 18.

Thus, in each of the water electrolysis cells 14, pure water iselectrolyzed by electricity, and hydrogen ions, electrons, and oxygenare produced. As a result, hydrogen is obtained on the cathode sidethrough coupling between the hydrogen ions and the electrons. Thehydrogen is taken out of the hydrogen outlet port 20 c to thehigh-pressure hydrogen pipe 42.

Meanwhile, oxygen (and permeation hydrogen) generated due to thereaction and unreacted water are flowing on the anode side, and amixture of these fluids is discharged from the water discharge port 20 bto the water discharge pipe 32. The unreacted water and the oxygen andhydrogen gases are introduced to the oxygen gas-liquid separationapparatus 28 and separated from one another. After that, the water isintroduced from the water supply pipe 22 to the water supply port 20 avia the circulation pump 26. The oxygen and the hydrogen separated fromthe water are discharged from the oxygen discharge pipe 38.

A hydrogen product (dry hydrogen) in a dry state is obtained by removingthe liquid water and the water vapor which are contained in the hydrogengenerated in the high differential pressure water electrolyzer 12. Afuel cell electric vehicle (not illustrated) is replenished with thishydrogen product. When replenishment of the above-described hydrogenproduct is completed, the operation of the water electrolysis system 10is stopped.

Next, a description is provided hereinbelow for a temperature controlmethod according to the embodiment with reference to a flowchartillustrated in FIG. 2 and a timing diagram illustrated in FIG. 3.

First, when the activation of the water electrolysis system 10 isstarted, the radiator fan 23 is stopped (step S1). In the controller 44,the second temperature sensor 48 has been monitoring the temperature ofdischarge water (discharge water temperature) discharged from the waterdischarge port 20 b of the high differential pressure water electrolyzer12 since the activation of the water electrolysis system 10 (step S2).Here, when it is determined that the temperature of discharge watermonitored by the second temperature sensor 48 exceeds a predeterminedtemperature Ta (YES at step S2), the processing proceeds to step S3.

As illustrated in FIG. 3, at the time of activation of the waterelectrolysis system 10, the discharge water temperature is higher than asupply water temperature because of self-heat generation by the highdifferential pressure water electrolyzer 12. This means that, bystopping the radiator fan 23, it is possible to rapidly increase thetemperature of the high differential pressure water electrolyzer 12.After the temperature of discharge water reaches the predeterminedtemperature Ta, which is a control temperature for a stack outlet (waterdischarge port 20 b), the radiator fan 23 is driven (step S3).

The predetermined temperature Ta is a temperature at which waterelectrolysis by the high differential pressure water electrolyzer 12reaches a stable state. To this end, in the controller 44, the firsttemperature sensor 46 monitors the temperature of supply water suppliedto the water supply port 20 a of the high differential pressure waterelectrolyzer 12 (step S4). Moreover, the processing proceeds to step S5,where proportional-integral-differential (PID) control is performedbased on the supply water temperature.

As illustrated in FIG. 3, in the PID control, the discharge watertemperature is higher than the supply water temperature by a temperaturedifference ΔT because of the self-heat generation by the highdifferential pressure water electrolyzer 12. In light of the above,driving of the radiator fan 23 is controlled such that the supply watertemperature detected by the first temperature sensor 46 is approximatedto a preset stack inlet (water supply port 20 a) control temperature Tb.As a result of this, the outlet temperature of the high differentialpressure water electrolyzer 12 is adjusted at a constant temperature.

The stack inlet control temperature Tb is a control temperature at whichto control the supply water temperature monitored by the firsttemperature sensor 46, the control temperature being based on estimationof the amount of heat generated in the high differential pressure waterelectrolyzer 12, and satisfying the condition that the discharge watertemperature monitored by the second temperature sensor 48 does notexceed a discharge water temperature threshold (boost activationtemperature) Tc.

In the controller 44, the second temperature sensor 48 monitors thedischarge water temperature. Additionally, when it is determined thatthe temperature of the discharge water exceeds the discharge watertemperature threshold (boost activation temperature) Tc (YES at stepS6), the processing proceeds to step S7. At step S7, the radiator fan 23is controlled at the maximum rotational rate to rapidly decrease thetemperature of water supplied to the high differential pressure waterelectrolyzer 12. Thereby, the outlet temperature of the highdifferential pressure water electrolyzer 12 is cooled down to atemperature below the discharge water temperature threshold Tc. Notethat the temperature of water may be decreased by increasing the flowrate of water compared to that in an ordinary state, in addition to thecase of controlling the radiator fan 23 at the maximum rotational rate.

Next, the processing proceeds to step S8, where it is determined whetheror not the system is to be stopped. When it is determined that thesystem is not to be stopped (NO at step S8), the processing returns tostep S5 and the PID control is continued.

In this case, in the embodiment, the water supply pipe 22 is providedwith the radiator 24. Besides, the first temperature sensor 46 and thesecond temperature sensor 48 are disposed upstream and downstream of thehigh differential pressure water electrolyzer 12, respectively. Thus, itis possible to adjust accurately and easily the temperature of watersupplied to the high differential pressure water electrolyzer 12.

Also, according to the embodiment, the second temperature sensor 48 hasbeen monitoring the temperature of discharge water discharged from thewater discharge port 20 b of the high differential pressure waterelectrolyzer 12 since the activation of the water electrolysis system10. During that period, it is possible to avoid unnecessary coolingcontrol by suspending the operation of the radiator fan 23 until thetemperature of the high differential pressure water electrolyzer 12increases to the predetermined temperature Ta due to the self-heatgeneration. This enables better improvement of the system efficiency(efficiency of water electrolysis).

Moreover, the second temperature sensor 48 is capable of preventing asituation where the temperature of the high differential pressure waterelectrolyzer 12 exceeds the upper limit by monitoring the outlettemperature of the high differential pressure water electrolyzer 12.Hence, it is possible to delay the deterioration of the highdifferential pressure water electrolyzer 12.

What is more, after water electrolysis by the high differential pressurewater electrolyzer 12 reaches the stable state, the first temperaturesensor 46 monitors the temperature of supply water supplied to the watersupply port 20 a of the high differential pressure water electrolyzer12. Here, driving of the radiator fan 23 is controlled and thetemperature of supply water is controlled.

For this reason, it is unlikely that an anomalous increase intemperature occurs after the high differential pressure waterelectrolyzer 12 shifts to a normal operation. Hence, if the temperatureof supply water is monitored, the temperature of water is controlledmore accurately. This eliminates the necessity of setting thetemperature threshold at normal operation to a low value in a safe zone,making it possible to effectively improve the efficiency of waterelectrolysis by the high differential pressure water electrolyzer 12.

Additionally, the radiator 24 is included as the cooling apparatus.Thus, with a simple and economical configuration, it is possible tobetter control the temperature of the high differential pressure waterelectrolyzer 12.

Furthermore, the stack inlet control temperature Tb is set, which is acontrol temperature at which to control the supply water temperaturemonitored by the first temperature sensor 46, the control temperaturebeing based on estimation of the amount of heat generated in the highdifferential pressure water electrolyzer 12, and satisfying thecondition that the discharge water temperature monitored by the secondtemperature sensor 48 does not exceed a discharge water temperaturethreshold Tc. Thus, it is possible to delay with certainty thedeterioration of the high differential pressure water electrolyzer 12attributed to the temperature exceeding the upper limit value.

Still further, the radiator fan 23 is controlled at the maximumrotational rate when the discharge water temperature monitored by thesecond temperature sensor 48 exceeds the discharge water temperaturethreshold Tc. This makes it possible to rapidly cool down the highdifferential pressure water electrolyzer 12 and to delay thedeterioration of the high differential pressure water electrolyzer 12.

A water electrolysis system according to the present disclosure includesa water electrolyzer, a water supply pipe, and a water discharge pipe.The water electrolyzer electrolyzes water to produce oxygen andhydrogen. The water supply pipe supplies water to a water supply port ofthe water electrolyzer, and the water discharge pipe allows the water,subjected to electrolysis, to be discharged from a water discharge portof the water electrolyzer.

The water supply pipe is provided with a cooling apparatus, and isprovided with a first temperature sensor which is located between thecooling apparatus and the water supply port and which monitors atemperature of supply water supplied to the water supply port. The waterdischarge pipe is provided with a second temperature sensor whichmonitors a temperature of discharge water discharged from the waterdischarge port.

Additionally, in this water electrolysis system, it is preferable thatthe cooling apparatus be a radiator.

The present disclosure further relates to a temperature control methodof a water electrolysis system. This temperature control method includesthe step of causing a second temperature sensor to monitor a temperatureof discharge water discharged from a water discharge port of a waterelectrolyzer from a time of activation of the water electrolysis system.This temperature control method includes the steps of, after thetemperature of the discharge water monitored by the second temperaturesensor exceeds a predetermined temperature, causing the firsttemperature sensor to monitor a temperature of supply water supplied tothe water supply port of the water electrolyzer, and starting control ofthe cooling apparatus to control the temperature of the supply water.

Still further, it is preferable that this temperature control methodinclude estimating an amount of heat generated in the waterelectrolyzer, and based on the estimation, setting a control temperatureat which to control the temperature of the supply water monitored by thefirst temperature sensor such that the temperature of the dischargewater monitored by the second temperature sensor does not exceed adischarge water temperature threshold.

Moreover, in this temperature control method, it is preferable that thecooling apparatus be a radiator.

What is more, in this temperature control method, it is preferable thata fan of the radiator be controlled at a maximum rotational rate whenthe temperature of the discharge water monitored by the secondtemperature sensor exceeds the discharge water temperature threshold.

According to the present disclosure, the water supply pipe is providedwith the cooling apparatus. Besides, the first temperature sensor andthe second temperature sensor are disposed upstream and downstream ofthe water electrolyzer, respectively. Thus, it is possible to adjustaccurately and easily the temperature of water supplied to the waterelectrolyzer.

Also, according to the present disclosure, the second temperature sensorhas been monitoring the temperature of discharge water discharged fromthe water discharge port of the water electrolyzer since the activationof the water electrolysis system. During that period, the coolingapparatus is suspended until the temperature of the water electrolyzerincreases to the predetermined temperature due to the self-heatgeneration. This makes it possible to avoid unnecessary cooling controland to better improve the system efficiency. Moreover, the secondtemperature sensor is capable of preventing a situation where thetemperature of the water electrolyzer exceeds the upper limit bymonitoring the outlet temperature of the water electrolyzer. Hence, itis possible to delay the deterioration of the water electrolyzer.

What is more, after the temperature of discharge water monitored by thesecond temperature sensor exceeds the predetermined temperature, thefirst temperature sensor monitors the temperature of supply watersupplied to the water supply port of the water electrolyzer, and controlof the cooling apparatus is started to control the temperature of thesupply water. For this reason, it is unlikely that an anomalous increasein temperature occurs after the water electrolyzer shifts to a normaloperation. Hence, if the temperature of supply water is monitored, thetemperature of water is controlled more accurately. This eliminates thenecessity of setting the temperature threshold at normal operation to alow value in a safe zone, making it possible to effectively improve theefficiency of water electrolysis.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. A water electrolysis system comprising: a waterelectrolyzer which electrolyzes water to produce oxygen and hydrogen; awater supply pipe which supplies the water to a water supply port of thewater electrolyzer; and a water discharge pipe which allows the water,subjected to electrolysis, to be discharged from a water discharge portof the water electrolyzer, wherein the water supply pipe is providedwith a cooling apparatus, and is provided with a first temperaturesensor which is located between the cooling apparatus and the watersupply port and which monitors a temperature of supply water supplied tothe water supply port, and the water discharge pipe is provided with asecond temperature sensor which monitors a temperature of dischargewater discharged from the water discharge port.
 2. The waterelectrolysis system according to claim 1, wherein the cooling apparatusis a radiator.
 3. A temperature control method of a water electrolysissystem including a water electrolyzer which electrolyzes water toproduce oxygen and hydrogen; a water supply pipe which supplies thewater to a water supply port of the water electrolyzer; and a waterdischarge pipe which allows the water, subjected to electrolysis, to bedischarged from a water discharge port of the water electrolyzer,wherein the water supply pipe is provided with a cooling apparatus, andis provided with a first temperature sensor located between the coolingapparatus and the water supply port, and the water discharge pipe isprovided with a second temperature sensor, the temperature controlmethod comprising the steps of: causing the second temperature sensor tomonitor a temperature of discharge water discharged from the waterdischarge port of the water electrolyzer from a time of activation ofthe water electrolysis system; and after the temperature of thedischarge water monitored by the second temperature sensor exceeds apredetermined temperature, causing the first temperature sensor tomonitor a temperature of supply water supplied to the water supply portof the water electrolyzer, and starting control of the cooling apparatusto control the temperature of the supply water.
 4. The temperaturecontrol method according to claim 3, the method further comprisingestimating an amount of heat generated in the water electrolyzer, andbased on the estimation, setting a control temperature at which tocontrol the temperature of the supply water monitored by the firsttemperature sensor such that the temperature of the discharge watermonitored by the second temperature sensor does not exceed a dischargewater temperature threshold.
 5. The temperature control method accordingto claim 4, wherein the cooling apparatus is a radiator.
 6. Thetemperature control method according to claim 5, wherein a fan of theradiator is controlled at a maximum rotational rate when the temperatureof the discharge water monitored by the second temperature sensorexceeds the discharge water temperature threshold.
 7. A waterelectrolysis system comprising: a water electrolyzer to electrolyzewater to produce oxygen and hydrogen, the water electrolyzer including awater supply port and a water discharge port; a water supply pipe whichis connected to the water supply port and via which the water is to besupplied to the water electrolyzer; a cooling apparatus provided in thewater supply pipe; a water discharge pipe which is connected to thewater discharge port and via which the water is to be discharged fromthe water electrolyzer; a first temperature sensor provided at the watersupply pipe between the cooling apparatus and the water supply port todetect a temperature of the water in the water supply pipe; and a secondtemperature sensor provided at the water discharge pipe to detect atemperature of the water in the water discharge pipe.
 8. The waterelectrolysis system according to claim 7, wherein the cooling apparatusincludes a radiator.
 9. A temperature control method of a waterelectrolysis system including a water electrolyzer to electrolyze waterto produce oxygen and hydrogen, the temperature control methodcomprising: detecting a temperature of water in a water discharge pipe,the water discharge pipe being connected to the water electrolyzer, thewater being discharged from the water electrolyzer via the waterdischarge pipe; detecting a temperature of the water in a water supplypipe between a cooling apparatus and the water electrolyzer after thetemperature of the water detected in the water discharge pipe exceeds apredetermined temperature, the water supply pipe being connected to thewater electrolyzer, the water being supplied to the water electrolyzervia the water supply pipe; and starting the cooling apparatus to controlthe temperature of the water in the water supply pipe, the coolingapparatus being provided in the water supply pipe.
 10. The temperaturecontrol method according to claim 9, further comprising: estimating anamount of heat generated in the water electrolyzer: and setting acontrol temperature at which to control the temperature of the water inthe water supply pipe based on the estimated amount of heat such thatthe temperature of the water in the water discharge pipe does not exceeda discharge water temperature threshold.
 11. The temperature controlmethod according to claim 10, wherein the cooling apparatus includes aradiator.
 12. The temperature control method according to claim 11,wherein a fan of the radiator is controlled at a maximum rotational ratewhen the temperature of the water in the water discharge pipe exceedsthe discharge water temperature threshold.